There are a number of solder bump forming methods according to the prior art. According to a plating method, metal is deposited on electrode pads of a microelectronic substrate through plating to form bumps. In another method typically referred to as a stencil printing method, solder paste, typically including flux, is printed onto electrode pads of a microelectronic substrate through a patterned stencil, and then, after stencil removal, the device is heated to melt the solder to form bumps therefrom. However, stencil printing sometimes results in solder paste lift-up by the stencil during stencil removal, causing significant fluctuations between solder paste amounts, on electrode pads of a substrate, which in turn lead to low volume solder bumps. The phenomenon of solder lift-up has been observed to become prevalent as solder resist opening (SRO) sizes and bump itches shrink. Solder lift-up poses problems such as missing bumps, and bum height variation in the substrate being processed for bumping, but also tends to contaminate the stencil, thus aggravating lift-up in subsequent printing processes using that same stencil. As a result of the above, it has been observed that the stencil printing method is not suited for high density interconnection structures, typically leading to high missing bump rates, and bump height variation, thus negatively affecting die attachment yields.
Different techniques have been introduced to address ever growing demands for pitch and SRO size reductions, such as pitches of about 160 microns and SRO sizes of about 80 to 90 microns. One such method involves the placement of micro balls or micro spheres of solder onto the electrode pads of a microelectronic substrate. According to an attachment mounting micro ball placement method, solder balls are sucked into a jig by vacuum suction and the solder balls then mounted onto flux-coated electrode pads of a microelectronic substrate. Another micro ball placement method involves the use of a stencil mask. According to the latter methods, solder balls are dispensed onto a ball alignment plate or stencil mask including openings therein in registration with electrode pads of a microelectronic substrate. A squeegee brush is then used to disperse the balls and press them into the mask openings. The electrode pads include flux thereon, which allows the balls to adhere thereto. The stencil mask is removed after ball placement. The solder balls are then heated and melted to form bumps.
Conventional micro ball placement poses problems however, among others where bumps are to be provided on a substrate having electrode pads and/or solder resist openings of differing sizes. A micro ball placement method, such as the attachment mounting method or the stencil mask method described above, typically results in solder bumps exhibiting significant bump height variations from bump to bump depending on the size of the electrode pad and/or solder resist opening used.
The prior art fails to provide a reliable method of providing solder bumps that do not exhibit problems typically associated with solder lift-up, such as, for example, missing bumps, bump voiding, and bump height variation. In addition, the prior art fails to provide a reliable method of providing solder bumps on electrode pads and/or solder resist openings of differing sizes on a microelectronic substrate.
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